US-Russian work tests sky-mapping techniques

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A US-Russian experiment involving MIT scientists has provided important data on a new technique to map the sky-specifically the ionosphere, or upper atmosphere. The data will help scientists from around the world refine the technique, which could become a tool useful for monitoring the ionospheric storms that can wreak havoc on satellites, for example.

Recent papers have discussed both the experiment itself, which among other things tested Russian and American approaches to the technique, and a severe ionospheric storm that the scientists observed during the experiment.

That storm, the result of a "solar bullet" of charged particles from the sun, "is of very high geophysical importance," said John C. Foster, assistant director of Haystack Observatory and a principal investigator for the work. As a result, he said, the experiment "was not just a technical success, it was a scientific success as well" because of the large amount of data that the scientists collected on the storm.

Scientists involved in the experiment are from Haystack, Moscow State University, the Polar Geophysical Institute in Murmansk, and the US Air Force's Phillips Laboratory in Bedford, MA.

The ionosphere is a highly variable medium found at an altitude of 100 to 1,000 kilometers. The Russian-American Tomography Experiment, or RATE, tested a technique called ionospheric radio tomography that could become an inexpensive way to map the ionosphere continuously on a global scale.

"With continuous coverage, you could call up a map of the ionosphere much like weather forecasters now call up weather maps," Dr. Foster said. Such an ability would greatly aid scientists' understanding of the ionosphere and could ultimately help them predict the storms there that affect radio signals, satellites and more. Dr. Foster noted that in recent months two Canadian satellites "were effectively destroyed" by ionospheric storms.

Ionospheric radio tomography involves a satellite that sends radio signals through the ionosphere to receivers located at intervals on the ground. By analyzing the radio signals once they reach Earth, scientists can determine variations in the density of the electrically charged gas that makes up the ionosphere. From there, they can map these variations to get the general structure of the ionosphere, including small-scale phenomena.

The technique could lead to global maps of the ionosphere because the receivers involved are small and portable, and could be distributed around the world. In contrast, the large radar facilities currently used to produce images of the ionosphere-there are six such facilities in the world, including the Millstone Hill Research Radar at Haystack-are "much more expensive to build and operate, precluding a large world-wide network," Dr. Foster said.

For RATE, which was conducted for 10 days in the fall of 1993, the scientists placed four receivers provided by the Russians in a north-south line along the northeastern US and eastern Canada. Russian navigation satellites flew over these sites every hour, sending down radio signals to all four receivers simultaneously. The resulting data were then analyzed to produce an image of the ionosphere using mathematical algorithms developed by the Russians.

Concurrently, the Air Force scientists placed US receivers at the same four sites and recorded signals from US satellites. They analyzed the data with their own set of algorithms.

The scientists then compared the American and Russian images produced via the experimental tomographic techniques to actual images of the ionosphere made over the same period from the Millstone Hill radar facility.

The result? Both the Russian and American tomographic images "compared very well to the Millstone Hill results," Dr. Foster said, though the Air Force results "were not quite as refined as the Russians.'" This is largely because the mathematical algorithms the Russian group used are more highly developed, Dr. Foster said. (The two groups have approached ionospheric radio tomography using mathematical techniques that are completely different.) However, Dr. Foster said, the Air Force receivers are more sophisticated than those of the Russians.

A paper on the work by 11 members of the RATE team was published last month in the International Journal of Imaging Systems and Technology. Haystack authors are Dr. Foster and principal research scientists Michael J. Buonsanto and John M. Holt. (Other RATE papers were presented at a conference in Wales last summer.)

The scientists are continuing to use the data to improve the satellite hardware and refine the mathematics. In addition, they are sharing the data with other scientists around the world who hope to refine their own tomographic algorithms (there are more than half a dozen different approaches toward analyzing the data, Dr. Foster said). After applying their algorithms to the data collected by the Russian and American receivers, these scientists can then cross-calibrate their own results by comparing them to the actual images produced at Millstone Hill.

"People are working with each other," he said. "Everyone is interested in making this technique work."

Because RATE coincided with a major magnetic storm in the atmosphere, the data it generated have great scientific interest. Several other scientific instruments that were operating simultaneously augment documentation on the storm. With that wealth of data, "we may be able to understand some new phenomena we observed related to magnetic storms," Dr. Foster said.

This month Dr. Foster presented a paper on the storm at a meeting of the Union of International Radio Scientists, where his overview of the storm included data collected "from the solar wind [the charged particles emitted by the sun that ultimately caused the storm] through the magnetosphere [the region around the Earth dominated by the planet's magnetic field] to the ionosphere."

The large amount of data on the storm will lead to "many more papers on the geophysics of what took place," Dr. Foster said.

A total of four Russian, 12 MIT, and four Phillips Laboratory researchers were involved in RATE. The MIT scientists most directly involved in the work were Drs. Foster and Holt at Haystack. Other Haystack researchers were Dr. Buonsanto, Dwight Sipler, Tab Gallardo, Aaron D. Pailes, Steve Sawicki, Chris E. Farrell, Alex P. Carson, Glenn Campbell, David Kotchman and Steve J. Cariglia.

The Russian principal investigators for the experiment were Professor Vyatcheslav E. Kunitsyn of Moscow State University and Professor Evgeny D. Tereshchenko of the Polar Geophysical Institute. The Phillips Laboratory principal investigator was John A. Klobuchar.

Participation of the Russian and MIT research teams involved in RATE was supported by the NSF; the Phillips Laboratory work was funded by the US Air Force. The Russian team acknowledges the Russian Ministry of Science for partial support of the research.

A version of this article appeared in MIT Tech Talk on January 25, 1995.